1. Field of the Invention
This invention relates to a method for the production of a cross-linked polymer. More particular, it relates to a method for the production of a cross-linked polymer for the formation of an absorbent resin. It further relates to an absorbent resin manifesting large absorption capacity without load and under load, suffering only a small deteriorated soluble component, and exhibiting excellent qualities when used as sanitary materials and a method for the production thereof.
2. Description of the Prior Art
In recent years, absorbent resins have been developed as polymers capable of gelating with a large amount of absorbed water and have been finding extensive utility in a wide range of applications as a sanitary absorbent agent for sanitary napkins and disposable diapers and as a water-retaining agent and a dewatering agent in the field of agriculture and horticulture and the field of civil engineering and construction industry.
As cross-linked polymers available for the formation of absorbent resins, partially neutralized cross-linked polyacrylic acid, modified cross-linked polyvinyl alcohol, cross-linked isobutylene-maleic anhydride copolymer, cross-linked polyethylene oxide, saponified acrylic ester-vinyl acetate copolymer, hydrolyzed starch-acrylonitrile graft polymer, and starch-acrylic acid graft polymer have been known.
As means for the production of these cross-linked polymers, such methods of reversed-phase suspension polymerization as disclosed in JP-A-56-161,408, JP-A-57-94,011, JP-A-57-158,209, and JP-A-57-198,714 and such methods of aqueous solution polymerization as disclosed in JP-B-02-19,122, JP-B-48-42,466, JP-A-58-49,714, JP-B-59-37,003, U.S. Pat. No. 4,286,082, and U.S. Pat. No. 4,625,001 have been known.
The methods of reversed-phase suspension polymerization, on account of inevitable use of an organic solvent, possibly harm the work environment and induce ignition and explosion, demand due measures, and prove costly because of the expense of the organic solvent and the expense required for the removal thereof. Since the organic solvent remains in a minute amount in the product, thorough removal of this residue forms an addition to the cost. Further, the cross-linked polymers which are obtained by the methods of the reversed-phase suspension polymerization have spherical shapes and small diameters. When they are used as for disposable diapers, therefore, they hardly remain fast on fibrous absorbent core components made of pulp and tend to fall off the core components and, therefore, allow no convenient handling.
By contrast, the methods of aqueous solution polymerization have none of the problems mentioned above. Specifically, the methods disclosed in JP-B-02-19,122, U.S. Pat. No. 4,625,001, etc. have been known, for example. The method disclosed in the patent publications mentioned above purports to produce a cross-linked polymer by subjecting the aqueous solution of a monomer destined to form a cross-linked structure and turn into a hydrogel polymer during the aqueous solution polymerization and a polymerization initiator to radical aqueous solution polymerization in a vessel provided with a rotary stirring shaft and carrying out this radical aqueous solution polymerization while finely dividing the hydrogel polymer formed in consequence of the advance of polymerization with the shearing force generated by the rotation of rotary arms or stirring vanes provided on the stirring shaft. These methods of production not only excel extremely in workability but also feature the ability to manufacture a finely divided hydrogel polymer possessed of a cross-linked structure in the molecule thereof with high productivity. Satisfactory conditions which enable these methods to produce a cross-linked polymer having a relatively large absorption capacity and an amply small content of soluble component have never been known. An attempt to obtain a cross-linked polymer having an amply small content of soluble component has resulted in a conspicuous decrease in productivity and an increase in the amount of residual monomer.
It is well known to persons of ordinary skill in the art that a decrease in the density of cross-links adds to the absorption capacity and, at the same time, increases the content of soluble component. The soluble component in the cross-linked polymer is such that, when the cross-linked polymer forms hydrogel on contact with such liquid as water, urine, or body fluid to be absorbed, it is exuded from the hydrogel. The soluble component which is extracted by the liquid being absorbed not only lowers the cross-linked polymer's absorption capacity but also aggravates the deterioration of the cross-linked polymer. Further, the ropiness of the soluble component brings about such unfavorable situations as emitting offensive feeling or polluting the liquid being absorbed.
The development of a method for producing a cross-linked polymer having a high absorption capacity and a small content of soluble component, therefore, has been longed for.
U.S. Pat. No. 4,654,039 and U.S. Pat. No. 4,985,514 have proposed methods for producing a cross-linked polymer having a high absorption capacity and a small content of soluble component by aqueous solution polymerizing a free acid type monomer or a monomer of a specific neutralizing ratio. These methods of production, however, suffer from inferior productivity because they produce an residual monomer in a large amount, necessitate neutralization as a subsequent step, or complicate the procedure.
U.S. Pat. No. 4,286,082 and U.S. Pat. No. 5,380,808 disclose methods for the production of a cross-linked polymer having a high absorption capacity and a small content of soluble component and a small amount of residual monomer by polymerizing the aqueous solution of a monomer while controlling a temperature of the polymerization system without stirring this aqueous solution. The fact that the step of polymerization is carried out without the action of stirring inevitably imposes a limit on the polymerization device to be used in performing the polymerization on a commercial scale. Then, the fact that the polymerization is performed while the temperature of the polymerization system is controlled in a prescribed range entails such problems as lowering productivity and adding conspicuously to the size of the polymerizing device.
The method which, by a convenient and expeditious process, produces a cross-linked polymer having a high absorption capacity and a small content of soluble component with high operational efficiency has not yet been established.
An object of this invention, therefore, is to provide a method of production which enables a cross-linked polymer having a high absorption capacity and a small content of soluble component to be produced with high operational efficiency.
As the characteristic properties which the cross-linked polymer is expected to possess as an absorbent resin, the high absorption capacity, the small content of soluble component, and the high absorption capacity under load may be cited. While the characteristic properties of the absorbent resin are stably retained for a long time when the absorbent resin is swelled with purified water or physiological saline solution, they are deteriorated with the elapse of time owing to an increase in the content of soluble component, a decrease in the absorption capacity without load and under load, and generation of conspicuously offensive feeling of ropiness when the absorbent resin is swelled with urine. Thus, the cross-linked polymer poses problems when it is used for such sanitary materials as disposable diapers.
Another object of this invention, therefore, is to provide an absorbent resin such that disposable diapers and other products which incorporate the absorbent resin manifest excellent qualities under the conditions of actual use.